Copper wire used in electronic applications is sometimes plated to protect the copper wire from corrosion. However, the plating material (e.g., nickel) will not easily form a proper intermolecular bond with solder, e.g., tin/lead type S, composition Sn60 or Sn63.
Criteria for acceptable bonding of the solder to the wire are set out in MIL-STD-202 Method 208, paragraph 4.6. Also set forth, in paragraphs 4.4 and 4.5, is a procedure for applying flux and solder to the wire. The procedure involves preparing the wire for subsequent solder application by cleaning the wire with a chemical flux, e.g., a rosin base flux (paragraph 4.4). The flux-treated wire is then immersed in a static solder pot containing molten solder (paragraph 4.5). In the case of plated wire, however, this method may not allow for proper cleaning of the plating, which, in turn, may not allow the solder to properly wet the wire. This problem is particularly pronounced with nickel plated wire. Wetting refers to the formation on the plating of a relatively uniform, smooth, unbroken and adherent film of solder.
An alternative method for bonding the solder to the wire involves the single step of immersing the wire in an ultrasonic solder pot, as disclosed in U.S. Pat. No. 3,912,544 to Sabatino, issued Oct. 14, 1975. The ultrasonic agitation improves the intermolecular bonding process over that of the static solder pot. In this process, the wire is immersed in the pot without having been first immersed in flux. However, manufacturers of ultrasonic pots recommend against placing wire with flux chemical thereon into the pot due to the fact that the flux may corrode the pot and cause its premature failure. Also, this ultrasonic process has been found unacceptable in meeting the solderability criteria of MIL-STD-202 Method 208, paragraph 4.6.